1. Field of the Invention
The present invention relates to a method for producing multi-level contacts, and more particularly, relates to a method which incorporates an anti-reflective coating (ARC) as a hard mask in production of multi-level contacts.
2. Description of the Prior Art
As integration density of integrated circuits increases, critical dimensions of devices become smaller and more demands on photolithographic and etching processes arise It is a challenge to shrink critical dimensions of devices so as to cary out flirther shrinking dimensions of next generation devices.
Near ultraviolet light or deep ultraviolet light is applied to expose and define patterns in photolithographic process and puts much influences on pattern resolution. From g-line (436 nm) and i-line (365 nm) to KrF (248 nm), light sources with decreasing wave lengths help to improve resolution and help to shrink critical dimensions of line width. However, light sources with decreasing wave lengths have narrower depths of focus and will result in poor exposure. Furthermore, swing curves of light intensity or line width versus depths of a photoresist increase in amplitudes and standing wave effects are amplified.
In producing multi-level contacts having shrunk critical dimensions, problems of light interference become even serious. Reflective light from multiple levels varies with line width and thus uniformity is deteriorated. Standing wave effects and notching even worsen the process. Furthermore, steps of etching for contacts with multiple levels are difficult to control.
In conventional methods for producing multi-level contacts, the line width of contacts varies, depending on the depths of contacts, because reflective lights from the first reflective surface travel or different distances. Light interference is more complicated. Contacts in the same exposure area may receive different exposure light intensities because the depths of contacts differ. Additionally, the shrunk dimensions of the line width increase light interference. In steps of etching and forming contacts after exposure, etching end points are hard to control since every multi-level contact has a different etching depth.
The conventional method for producing multi-level contacts is described hereinafter with reference to FIG. 1. A masking photoresist 12 is patterned on a dielectric layer 13 which is superimposed on a silicon substrate 11. In general, the dielectric layer 13 is made of a borophosphosilicate glass (BPSG), tetraethyl-orthosilicate (TEOS) or other dielectric materials. Devices which are defined respectively by an isolation layer 14, tungsten silicide 15 and polysilicon 16 are formed on the substrate 11. An incident light 17 is projected onto the dielectric layer 13 via an unmasked area and part of the light 17 arrives at the devices and then is reflected. The reflected light either exposes the photoresist 12 or interferes with the incident light 17. Standing wave effects occur and poor profiles are thus made. In the step of etching after exposure, etching end points are hard to control. If the etching step stops at a deep position, photoresist loss may occur and devices at a high level may be excessively etched and damaged. However, if the etching step stops at a less deep position, devices at a low level may have residues thereon and may have high contact resistance.
U.S. Pat. No. 4,758,305 uses multi-level photoresist, over-exposure and over-development to produce a spacer wall in the formation of contacts which can be applied for 0.8 .mu.m to 1.25 .mu.m patterns. Not any hard mask is required. The contacts thus formed are with one single depth. The problems of multi-level contacts are not solved or even raised. U.S. Pat. No. 5,355,020 discloses semiconductor devices having multi-level metal contacts in which planarization and anti-reflection techniques are applied. The `multi-level` in the multi-level metal contacts refers to a multiple layer of metal lines, and the planarization and anti-reflection techniques are applied in production of metal lines.
For difficulties of control in contact hole diameters, which are caused by the multi-level contacts having contact holes of various depths, conventional methods do not provide solutions.